Conventionally, in a cutting tool having a plurality of throw away tips, if chipping or abnormal wearing is caused even in one tip, vibration is inherently caused during machining of a work to thereby make it difficult to machine with high precision, and excessive load can be exerted on other normal tips to thereby cause chipping or abnormal wearing even in the normal tips.
Therefore, various detecting methods or detecting systems for detecting abnormality, such as chipping or so forth, caused during cutting work have been proposed in the prior art and put into practical use.
For example, in Japanese Unexamined Patent Publication No. Showa 61-111877 or Japanese Examined Patent Publication No. Heisei 1-33299, "Edge Chipping Detector" or "Abnormality Detecting Method of Cutting Tool" detecting a vibration caused at the occurrence of chipping of the edge during cutting work and thus detecting abnormality of the cutting tool, have been proposed.
On the other hand, in Japanese Unexamined Patent Publication No. Showa 61-252051, there has been proposed an "Edge Chipping Detecting System" optically detecting chipped edge by a signal from the edge detected by a photoelectric switch with an optical fiber and a signal from an edge mounting position detecting circuit.
Furthermore, in Japanese Unexamined Patent Publication No. Heisei 6-39685, there has been proposed a "Tool Damaging Detecting System of Cutting Tool" detecting wearing or chipping of an edge by means of an image pick-up means, such as an optical sensor, TV camera or so forth.
In the abnormality detecting method or the abnormality detecting system detecting abnormality of the edge from vibration caused by chipping or wearing of the edge as disclosed in the foregoing Japanese Unexamined Patent Publication No. Showa 61-111877 or Japanese Examined Patent Publication No. Heisei 1-33299, a drawback is encountered in that when a cutting amount is small such as in finish machining, the vibration caused when chipping of the edge occurs may only be slight, thereby making it difficult to detect abnormality of the edge.
On the other hand, in the cutting tool provided with a plurality of rows of edges, such as a turn broach, vibration caused due to abnormality of the edge is superimposed thereby/making it extremely difficult to identify the row in which abnormality of the edge is caused.
Therefore, a drawback is encountered in that when abnormality is detected, the damaged edge has to be visually found among several hundreds of edges, which requires a substantial work load in identifying the damaged edge.
On the other hand, in the abnormality detecting systems that optically detect abnormality of the edge by means of a photoelectric switch with an optical fiber or an image pick-up means as disclosed in Japanese Unexamined Patent Publication No. Showa 61-252051 or Japanese Unexamined Patent Publication No. Heisei 6-39685, a drawback is encountered in that since it is difficult to detect abnormality of a fine edge, when precision in detection is low and the system detects reflection light from the edge, it becomes impossible to detect abnormality for significant lowering of reflection index of the light when the surface of the edge is oxidized and blackened due to the heat generated during cutting work.
Also, there is a method which takes an image of each tip by means of the image pick-up means and makes judgement of abnormality of each tip by processing the image. In this method, accurate abnormality detection becomes impossible due to an error in the position on the screen as being subjected to the influence of thermal distortion for heat generation of the overall machine during machining of the work or as being influenced by error of the mounting position which may be caused upon re-mounting of the image pick-up means or the tool after removal for maintenance.
For this reason, in the prior art, the image of the tip taken by the image pick-up means is processed by pattern matching to perform position recognition to recognize the position of the tip on the screen. However, when the surface of the edge is oxidized and blackened by the heat generated during cutting work as set forth above, the following drawback is caused.
Normally, this type of overall tip C is colored by gold or the like. When cutting work is performed, a part of the edge is oxidized and blackened as shown in FIG. 1A due to the heat generated during cutting.
When the image of the tip C, the edge of which is blackened, is picked up by the image pick-up means and an inspecting region is set to the edge portion of the tip C, a normal portion Ca of the gold color and a blackened portion Cb are present in admixed fashion in the inspecting region.
On the other hand, when a defective portion Cc, due to chipping of the edge or so forth, is caused in the blackened portion Cb in the inspecting region, a raw metal of the tip C is exposed in the defective portion Cc. Therefore, a luminance distribution of the inspecting region becomes as shown in FIG. 1B to have an equivalent luminance value as the normal portion Ca in the defective portion Cc.
Here, when the lighting for picking-up of the image is weak, if abnormality is judged with a threshold value set at a position T1 of FIG. 1B, a drawback is encountered in that the normal portion Ca showing a higher luminance value than the threshold value T1 can be judged as being abnormal to make it impossible to accurately judge abnormality of the tool.
When the threshold value is set at a position T2 of FIG. 1B in order to prevent such defects, it can be avoided to judge the normal portion Ca as being abnormal. However, the number of extracted pixels having the luminance value exceeding the threshold value T2 of the defective portion Cc is reduced significantly. As a result, a drawback is encountered in that a fluctuation of the luminance value is caused due to fluctuation of lighting or stain depositing on the edge, and defective portion Cc cannot be recognized even when the defective portion Cc is caused in the inspecting region, to significantly lower accuracy of detection.
The present invention has been worked out for solving such drawbacks. And it is an object of the present invention to provide a tool abnormality detecting method for a machine tool which can accurately detect abnormality, such as chipping of the edge caused in an inspecting region of the tip and a tool abnormality detecting system for a machine tool which can recognize the position of each tip based on a reference mark taken together with an image of the tip.